In the manufacture of microelectronic packages called hybrids, or multi chip modules, unpackaged semiconductor dice are mounted face down on a substrate such as a printed circuit board. This type of mounting is referred to as flip chip mounting because each unpackaged die is flipped circuit side down onto the substrate. With flip chip mounting the bond pads on the die are in direct electrical contact with corresponding contact pads on the substrate.
One method of attaching a flip chip mounted semiconductor die to a substrate employs anisotropic adhesives. An anisotropic adhesive takes the place of conventional solder and socket connections for securing the die to the substrate and also for establishing an electrical pathway.
Anisotropic adhesives are provided in a variety of forms. Anisotropic adhesives are formed such that electrical resistance in one direction through the material will differ from that measured in another direction. Typically high resistance is provided in at least one orthogonal direction (e.g., x-y direction) while low resistance is provided in the remaining orthogonal direction (e.g., z direction). The conductivity in one direction is typically pressure sensitive requiring that the material be compressed in that direction.
One type of conductive anisotropic adhesive is known as a z-axis anisotropic adhesive. Z-axis anisotropic adhesives are filled with conductive particles to a low level such that the particles do not contact each other in the x-y plane. Compression in the z-direction establishes an electrical path. Z-axis anisotropic adhesives are formed as a viscous paste or as a film that is applied and then cured to harden. Both types of z-axis anisotropic adhesives (film or paste) come in a thermoplastic or thermosetting variety. Thermoplastic anisotropic adhesives are heated to soften for use and then cooled for curing. Thermoset anisotropic adhesives require heat curing. Using an anisotropic adhesive, a die can be adhered to a substrate and multiple connections can be made to the bond pads on the die with a single piece of material.
One problem with flip chip mounting using z-axis anisotropic adhesives is that the conductivity of the material is pressure sensitive. Accordingly the anisotropic adhesive must be compressed during the curing process to establish a permanent electrical connection through the material. In the past, compression of the z-axis anisotropic adhesive has been accomplished by maintaining a load on the die during the curing process. This load is typically applied using some type of mechanical tooling fixture.
One problem with this type of arrangement is that mechanical fixtures tend to be complex and expensive. Mechanical fixtures also have a tendency to damage the die or substrate. In addition, the compressive load must be maintained for a relatively long time period, which is required to sufficiently cure the elastomer (e.g., twenty to thirty minutes). For a high volume manufacturing process this type of curing process is unacceptably long. Additionally, if more than one flip chip is to be attached, a sequential process is used in which one chip is attached and cured, followed by attachment and curing of subsequent chips. This sequential process can result in multiple high temperature exposure of some of the chips.
In view of the foregoing, it is an object of the present invention to provide an improved method and apparatus for mounting components to a substrate using anisotropic adhesives. It is a further object of the present invention to provide an improved method and apparatus for flip chip mounting semiconductor dice to a substrate. It is yet another object of the present invention to provide an improved method and apparatus for flip chip mounting semiconductor dice that are compatible with high volume manufacturing operations. Other objects, advantages and capabilities of the present invention will become more apparent as the description proceeds.